Release of hormones and transmitters from endocrine cells and nerve endings is believed to occur by exocytosis. While the molecular basis of this process remains obscure, it is known that isolated secretory vesicles from brain and adrenal medulla can be induced to release their contents by exposure to ATP, Mg ion, Ca ion and other ions. This reaction is viewed as a model of chemical events leading to vesicle fusion with the plasma membrane and subsequent fission. We have found in the adrenal that ATP analogues blocked at the beta-gamma position with -NH- or -CH2- can also induce release. These analogues are not cleaved by ATPase, and these results suggest that the motive force for release is not cleavage of ATP. We have also found that an intact alpha-beta oxygen bond in ATP is required for release. Cleavage at this bond occurs in the adenyl cyclase reaction, and we have found this enzyme to be intrinsic to the vesicle membrane. Furthermore, it is activated only when the granule releases its contents. We conclude that ATP may be inducing release by acting as a cyclase substrate. In separate studies we have also found that isolated vesicles have a transmembrane potential, sensitive only to protons and ATP. ATP analogues that induce release can also induce depolarization of the vesicle. We conclude that depolarization of the vesicle as well as the synthesis of cAMP, may be coupled to release.